Pitch in a papermaking system can be simply defined as the sticky, resinous material that is released from wood during the pulping process. In paper mill process waters, pitch exists as an unstable, colloidal dispersion of hydrophobic particles. Under the conditions often encountered in a papermaking system, such as hydrodynamic and mechanical shear forces, abrupt pH and temperature changes and exposure to water hardness ions and inorganic scale deposits, colloidal pitch particles tend to agglomerate and deposit on paper machine surfaces.
Pitch deposits often lead to quality defects in the finished paper product, shortened equipment life, impaired system operation, paper machine downtime and, ultimately, lost profits for the mill. These problems are magnified when a paper mill "closes up" its process water system, as many mills have already done for conservation and environmental reasons, thus eliminating many potential exit points for pitch in the system. A closed, recirculating papermaking process water system only has a limited holding capacity for hydrophobic materials like pitch. Unless these pitch particles are continuously removed from the system in a controlled manner, spontaneous system purges can occur which lead to pitch deposits and runnability problems. Thus, the control of pitch deposition in a papermaking system is a priority for many papermakers.
A number of pitch deposit control methods are used in the paper industry. For example, optimizing the performance of the pulp washing stages (e.g., kraft brown stock washers and bleach plant extraction stages) through the application of pitch dispersants and defoamers or washaids to these stages is a control option for many mills. The removal of pitch through these viable exit points is especially important in closed papermaking systems. The use of pitch adsorbants such as talc is often employed; however, unless the talc/pitch particles are effectively retained in the paper sheet, talc can end up contributing to, rather than solving, the pitch deposit problems.
Alum is a widely used pitch control agent for acid papermaking systems. It acts to attach pitch particles to fibers in a manner analogous to the setting of rosin size. Cationic coagulants promote the attachment of the anionically charged, colloidal pitch particles to fibers and fines through a charge neutralization mechanism. The advantage to using cationic coagulants and alum for pitch control is that pitch is removed from the system in the form of microscopic particles dispersed among the fibers in the finished paper product. Unlike alum, a polymer's cationic charge is not necessarily dependent on the pH of the system, thus cationic polymers can be used effectively in neutral and alkaline paper machines. In addition, cationic polymers remain soluble under normal alkaline papermaking conditions while alum can form insoluble aluminum hydroxide.
The present inventors undertook the task of examining the effects of polymer charge, chemistry and molecular weight for various polymers to determine their performance in controlling pitch in papermaking systems. As such, the present inventors discovered that hydrophobically modified copolymers of DADMAC and DMAEA or DMAEM are good agents for the removal or control of pitch in pulp and paper mill processes. Particularly effective copolymers were diallyldimethylammonium chloride/dimethylaminoethylacrylate benzyl chloride quaternary (DADMAC/DMAEA.BCQ) and diallyldimethylammonium chloride/dimethylaminoethylmethacrylate cetyl chloride quaternary (DADMAC/DMAEM.CCQ).
The present invention also provides many additional advantages which shall become apparent as described below.